The present invention relates to a synergistic composition for dissolving metal oxides and in particular iron oxides.
The dissolution of iron oxides is a very important technological process with applications in a number of different fields. Effective dissolution agents can be useful for (1) leaching of oxide ores in hydrometallurgy to recover metal values from the oxide ores; (2) removal of iron oxide contaminants from non-metallic materials such as minerals (e.g. kaolin and silica), of importance for the glass and ceramic manufacturing industry; (3) stripping of metal ions from extraction solvents or from ion exchange resins; (4) cleaning of metals prior to subsequent processing (e.g. removal of scale from a metal prior to rolling, anodizing, galvanizing or painting; (5) removal of deposits from equipment in contact with hot water or steam (e.g. thermal power equipment, heat exchangers and piping, and boilers); (6) cleaning or treatment of rusty bolts, nuts and connectors; (7) removal of stains (iron oxide related) from fabrics; and (8) treatment of radioactive materials including removal of a surface layer of oxides from steel equipment used for radioactive material processing or in nuclear plants, in order to facilitate the removal of radioactive contaminants (e.g. the actinide and lanthanide elements) present in or below the surface oxide layers by subsequent application of an appropriate complexing agent; leaching of radioactive sludge; and leaching of radioactive soil.
It is known in the art that acids accelerate the dissolution of metal oxides; however severe conditions of concentration or temperature are usually required to achieve fast rates. These conditions present safety hazards in handling and the potential for corrosion, if metals are present.
It is also known in the art that reductants can accelerate the dissolution of metal oxides, but the dissolution rates under moderate conditions are still low.
In general this can be seen in the following:
GB-1229582 ((Albright & Wilson Mfg. Ltd.-1971)- U.S. Pat. No. 3,634,257 (1972) equivalent) describes the use of phosphonic acids to remove encrustations containing calcium carbonate or iron compounds from rigid surfaces, e.g. stills and boiler tubes. The preferred acid is 1-hydroxyethylidene-1,1-diphosphonic acid, which can also be used with other scale removing acids and with ammonia.
Albright & Wilson sells a product based on GB 1229582, BRIQUEST DP 13, for use as a railway carriage cleaner. It is described as a blend of BRIQUEST ADPA (1-hydroxyethylidene-1,1-diphosphonic acid) and orthophosphoric acid specifically formulated for use with carriage-washing machines.
Valverde N. and Wagner, C., Ber. Bunsenges Physik. Chem. 80(4) (1976) 330 and Valverde N., Ber. Bunsenges. Physik. Chem. 80(4) (1976) 333 describe experiments which show that the dissolution of iron oxide is accelerated by decreasing the redox potential and attribute this to reduction of Fe.sup.+3 in the oxide lattice to give the more soluble Fe.sup.+2.
Lu, Z. Y. and Muir, D. M., Hydrometallurgy 21 (1988) 9 describes experiments in which an enhanced leaching rate of metal ferrites and iron oxides is found when HCl leaching solutions also contain Cu.sup.+1 or Sn.sup.+2 as reductants.
Zinder, B., Furrer, G. and Stomm, W., Geochimica et Cosmochimica Acta 50 (1986) 1861 describes acceleration of the rate of alpha-FeOOH dissolution by NaNO.sub.3 in the presence of ascorbic acid.
Surana, V. S. and Warren, H. J., Trans. Inst. Min. Metal. Sec. C, 28 (1969) C133-C139 and Kunda, W., Rudyk, B. and Mackiw, V. N., Bull. Can. Inst. Min. Metall. 61 (1968) 819 describe the use of SO.sub.2 in dissolution of iron oxides.
Warren, I. H., "Removal of Iron Oxide from Silicate Minerals" in M. E. Wadsworth and F. T. Davis (Eds.), Unit Processes in Hydrometallurgy, Vol. 1, pages 300-307, A.I.M.E., NY, 1964 describes the use of sodium dithionite in dissolution of iron oxides.
Gorichev, I. G. and Kipriyanov, N. A., "Regular Kinetic Features of the dissolution of Metal Oxides in Acidic Media", Russian Chemical Reviews 53(11) (1984) 1039 is a review article which discusses the effect of redox couples on the rate of dissolution of metal oxides.
Azuma, K. and Kametani, H., Trans. Metall., Soc. AIME 230 (1964) 853 indicates that the rate of dissolution of iron oxides increases with the stability constant of Fe.sup.+3 with the anion of the electrolyte.
Banwart, S., Davies, S. and Strumm, W., Colloids and Surfaces 39 (1989) 303, describes the use of oxalic acid and L-ascorbic acid to dissolve iron oxide and shows an advantage for using the two together.
Gorichev, I. V., Gorsheneva, V. F., Kipriyanov, N. A. and Klyuchnikov, N. G., Kinetika i Kataliz 21 (6) (1980) 1422, describes the dissolution of magnetite (Fe.sub.3 O.sub.4) with 1-hydroxyethylidene-1,1-diphosphonic acid and refers to the potential use in removal of magnetite deposits and scale from steel.
U.S. Pat. No. 3,854,996 to Frost teaches the use of polyphosphonic acids to remove magnetite (iron oxide) scale. To help with the action, the use of an oxidizing agent is recommended.